short on/off cycle times for burner/inducer

Heil natural gas furnace Model #NUGK075DG07 - 867.769426. Years ago I replaced our old murcury thermostat with a Honeywell CT3200. We noticed the furnace cycled more often but thought it was related to efficiency of the new thermostat. I'm having second thoughts after monitoring the on/off time of the burners/inducer blower (exhaust), as well as the air circulation blower. Over 1 hour 30 minutes, the burners and inducer motor ran for 1 minute 15 seconds (average), and turned off for 1 minute 15 seconds (average). +/- 5 seconds every time. Over the same period, the air circulation blower turned
OFF 1:48 (minutes:seconds)
ON 23:42
OFF 2:06
ON 17:14
OFF 2:28
ON 17:06
OFF 1:58
ON 13:00
OFF 2:38

The furnace manual says to allow the furnace to run for 10 minutes then inspect the main burner for stable soft blue flames. ("Dust may cause orange tips or they may have wisps of yellow, but they must not have solid yellow tips. No curling, floating, or lifting off, and must not touch the sides of the heat exchanger"). In my case the burners don't stay on for 10 minutes, although the air circulation blower does. The internal flame tips are solid blue, the external flames have intermittent yellow wisps most of the time, just not solid yellow all the time. Here is the 12 second video for a better visual:

Are these cycle times normal or is this just an indication that the burners and/or heat exchanger need to be cleaned? Is this something a mechanically inclined person can handle after shutting off the gas and power? (I did have the gas turned off last Saturday to replace a water heater - will air in the gas lines cause this?)

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short on/off cycle times for burner/inducer

One other thing I found....
The furnace user manual mentions you might be able to adjust the thermostat heat anticipator if you are experiencing short on cycles. The thermostat manual says, "the thermostat on-time is factory set for a warm air, gas or oil heating system (screw A & B in, which is currently the case). But it also says "Important - When using a high efficiency furnace such as a 90% or greater AFUE (Average Fuel Utilization Efficiency) unit, adjust screw A out one turn and leave screw B in." I can't find any efficiency rating in the user manual or on the furnace. When we purchased the house the realtor said it was installed in 1991.

Does anyone know if this unit is 90% efficient?
Is the thermostat adjustment safe to try without damaging anything?

short on/off cycle times for burner/inducer

Beenthere (or anyone else for that matter) - you mentioned the short burner cycle times could be from a dirty coil. I'm not sure if I have one, since I don't have central air (evaporator coil) and the furnace is gas not electric (electric heating coil).

short on/off cycle times for burner/inducer

I'm using an AirMedic Plus Permanent Electrostatic Air Cleaner. I cleaned it about a month ago, as well as tonight. I take it apart, rinse the three layers with water and let it dry overnight (while drying I use a cheap fiberglass filter). I believe it's a 3 stage filter simply because there's a foam type filter between an outside layer of nylon mesh fabric on each side. See the pictures...

short on/off cycle times for burner/inducer

Ok I replaced the electrostatic filter with a cheap 16x25x1 fiberglass filter last night. The cycle times didn't change much with the fiberglass filter, however it is 15 degrees warmer outside tonight, compared to the numbers in my first post. Both sets of numbers below in Microsoft Word attachment.. Just to be clear the inducer/burners cycle on/off about 7 times while the circulation blower is running. The circ. blower may run for 18 minutes before cycling off (it decreases the closer it get tot he target temp). I'm not having the problem where the burners shut off when the blower starts.

I did notice slightly longer on time (plus 7-10 seconds) & shorter off time (minus 10-15 seconds) for the vent/burner. Except when the target temp was reached and the circ. blower shut off, the vent/burner off time was much higher.. six minutes plus, but probably due to a higher outside temp. The air circulation blower had slightly shorter on cycles and longer off cycles. Overall I think it's still short cycling. Maybe I have to get comparison numbers when the outside temp is similar. What do you guys think?

By the way, today I found out from the distributor this furnace is a 90% AFUE unit. (I was surprised since it was installed in 1991). From my post above: The furnace user manual mentions you might be able to adjust the thermostat heat anticipator if you are experiencing short on cycles. The thermostat manual says, "the thermostat on-time is factory set for a warm air, gas or oil heating system (screw A & B in on back). That's my setup now! But it also says "Important - When using a high efficiency furnace such as a 90% or greater AFUE (Average Fuel Utilization Efficiency) unit, adjust screw A out one turn and leave screw B in." I can give that a try too.

short on/off cycle times for burner/inducer

short on/off cycle times for burner/inducer

The FAF is a 1991 mod ??? Did anyone mention mabe a cracked heat exchanger ??? mabe cycling on the limit

still could be t. stat. If the wires are connected correctly. You could ( at the t. stat connect the red & white wires together ( this takes the t.stat out of circuit ))& see if the furnace stays on. If so replace the t.stat

short on/off cycle times for burner/inducer

The burner cycling on off like that is not generally caused by the stat.

Can you check the air temp at the furnace supply.
Or at least at the register closest to the furnace.

I bumped the thermostat up 2 degrees (66-68). At the furnace supply vent, the burner cycled on/off at 140/160 degrees. At the filter the temp slowly climbed from 62-65 degrees. Then the stat shut the blower down at the desired 68 degree room temp. The tag on the furnace says the "Designed Max Outlet Air Temp is 165 degrees". Also the "system on" indicator at the thermostat is always on while warming up the house - it's not cycling, unless the desired temp is reached and the blower shuts down. Just the burner is cycling at the temps above.

It appears to me, and I certainly could be wrong, that the burners are cycling at the proper high temp limit then. And I could have an oversized furnace (Input BTU/HR 75000, Alt Input BTU/HR 60000, 1388 sq ft gross living area) or maybe there isn't enough supply/return duct to the house. Supply: basement has 2 vents, 1st floor has 6 vents, 2nd floor has 4 vents. Return: basement has 1 vent at furnace, 1st floor has 3 vents, 2nd floor has 0 vents. What do you guys think?

Last edited by diggerdave; 02-26-2009 at 01:04 PM.
Reason: Added more info